1. Field of the Invention
This invention relates generally to aircraft braking systems, and more particularly concerns a method and system for antiskid control of brake assemblies of an aircraft, utilizing a discrete Kalman filter combined with reference velocity and integrator modules. The present new optimal controller compels the wheel velocity to quickly converge to the reference velocity.
2. Description of Related Art
A conventional skid detection system used in aircraft braking systems typically includes a wheel speed transducer for each wheel brake of the wheels of the aircraft, for measuring wheel speed and generating wheel speed signals that are a function of the rotational speed of the brake wheel. The wheel speed signal is typically converted to a signal representing the velocity of the aircraft, and compared with a desired reference velocity, to generate wheel velocity error signals indicative of the difference between the wheel velocity signals from each braked wheel and the reference velocity signal. The output of the velocity comparator is referred to as velocity error. The velocity error signals are adjusted by a pressure bias modulator (PBM) integrator, a proportional control unit, and a compensation network, and the outputs of these are summed to provide an antiskid control signal received by the command processor. The PBM integrator in the antiskid loop dictates the maximum allowable control pressure level during braking. When no skid is detected, this integrator allows full system pressure to the brakes.
The conventional PID controller for aircraft brake control systems deals with various conditions such as aerodynamics, landing gear dynamics, xcexc-slip profile, different landing conditions, and the like. One major problem is that tuning of controller parameters to guarantee high efficiency in different landing conditions and conditions affecting the tire-runway coefficient of friction (xcexc) of the aircraft braking system is often a difficult task.
In the modern state space optimal controller design for the aircraft brake control systems, some efforts to utilize Kalman filter techniques are noted that attempt to provide some advantages such as an estimation of a peak in a mu-slip curve of the tire-runway coefficient of friction, estimation of the optimal slip value, and the like. However, control system observability and controllability continue to be a problem with complex tire-runway xcexc conditions, aerodynamics, nonlinear landing gear dynamics, various landing conditions, and the like, and successful simulation results have not been demonstrated even in extremely simplified dynamic models. Therefore, it would be desirable to provide a new optimal antiskid brake control method and system implementing an appropriate Kalman type optimal controller to provide an improved antiskid brake efficiency and improved braking control tuning in combination with a conventional PBM integrator for aircraft brake control systems. The present invention meets these and other needs.
Briefly, and in general terms, the present invention provides for an improved system and method for antiskid braking control implementing an optimal filter control methodology in combination with a conventional PBM aircraft brake integral control system. An optimal antiskid braking controller is provided that includes a wheel speed filter, a reference velocity module, an optimal controller, and a PBM integrator module. The optimal controller includes a discrete Kalman regulator utilizing a Kalman filter, which compels the wheel velocity to quickly converge to the reference velocity, while the integrator module produces appropriate antiskidding control and compensates for low frequency torque disturbances. The optimal controller estimates brake pressure, and the difference between wheel velocity and a reference velocity, and these estimated states are regulated by a control feedback gain matrix. The weighting matrices and all gains are precalculated, and performance of the controller can be improved by adjustment of these factors. The system and method of the invention can also be applied to an electric brake control system with slight modification of dynamic model parameters, gain values and weighting factors. The optimal antiskid braking controller of the invention allows for improvement of antiskid brake efficiency, and improvements in controlled tuning of the conventional PID aircraft brake control system.
The invention accordingly provides for an improvement in a system for controlling braking of an aircraft during landing, the system including wheel velocity signal generating means for producing a wheel velocity signal that is a function of the rotational speed of the wheel, and means for measuring brake torque applied to a wheel brake of the aircraft. Typically, the wheel velocity signal generating means comprises a wheel speed filter for generating a filtered wheel velocity signal based upon the wheel velocity signal. The system for controlling braking of an aircraft during landing includes a velocity reference generator for generating a reference velocity signal indicating a desired reference velocity, aircraft velocity comparison means for comparing the wheel velocity signal with the reference velocity signal for generating a velocity error signal indicative of the difference between the aircraft wheel velocity signal and the reference velocity signal, and an optimal brake controller for generating an optimal brake pressure control signal for the wheel of the aircraft to cause the aircraft wheel velocity to converge to the reference velocity, based upon an estimated command brake pressure and an estimated value of a coefficient of friction between the tire and runway surface. A pressure bias modulator integrator is provided that is responsive to the wheel velocity signal and the reference velocity signal to provide an antiskid control signal, and means are provided for summing the optimal braking command signal and the antiskid control signal to produce the command brake pressure signal.
In a presently preferred embodiment, the optimal brake controller comprises a discrete Kalman regulator for determining the estimated command brake pressure and the estimated value of the coefficient of friction. The discrete Kalman regulator, in a preferred aspect of the invention, comprises a control feedback gain matrix and a Kalman filter, the Kalman filter receiving the velocity error signal and a brake torque feedback signal, and the Kalman filter generating an estimated velocity error signal and an estimated brake pressure, and the control feedback gain matrix receives the estimated velocity error signal and the estimated brake pressure to generate the estimated command brake pressure and the estimated value of the coefficient of friction between the tire and runway surface. In another presently preferred aspect, the optimal brake controller determines the optimal brake pressure control signal based upon the estimated value of the coefficient of friction between the tire and runway surface, the weight per wheel, the rolling radius of a tire, the reciprocal of the torque vs. pressure ratio, and the estimated command brake pressure.
The present invention similarly provides for an improvement in a method for controlling braking of an aircraft during landing. The method includes the steps of generating a wheel velocity signal that is a function of the rotational speed of a wheel of the aircraft, and applying a command brake torque signal based upon a command brake pressure to the wheel brake of the aircraft. The method also typically involves filtering the wheel velocity signal to generate a filtered wheel velocity signal. A reference velocity signal is generated indicating a desired reference velocity, the wheel velocity signal is compared with the reference velocity signal for generating a velocity error signal indicative of the difference between the aircraft wheel velocity signal and the reference velocity signal, and an optimal brake pressure control signal is generated for the wheels of the aircraft to cause the aircraft wheel velocity to converge to the reference velocity, based upon an estimated command brake pressure and an estimated value of a coefficient of friction between the wheel and runway. The estimated command brake pressure and the estimated value of the coefficient of friction are determined with a discrete Kalman regulator, an antiskid control signal is provided; and the optimal braking command signal is summed with the antiskid control signal to produce a command brake pressure signal.
In a presently preferred embodiment of the method, the discrete Kalman regulator comprises a control feedback gain matrix and a Kalman filter, the Kalman filter receives the velocity error signal and a brake torque feedback signal, and the Kalman filter generates an estimated velocity error signal and an estimated brake pressure. The control feedback gain matrix receives the estimated velocity error signal and the estimated brake pressure to generate the estimated command brake pressure and the estimated value of the coefficient of friction. In a preferred aspect, the step of determining the optimal brake pressure control signal is based upon the estimated value of the coefficient of friction, the weight per wheel, the rolling radius of a tire, the reciprocal of the torque vs. pressure ratio, and the estimated command brake pressure.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.